1. Field of the Invention
This invention is in the field including ultra high frequency transmission lines and more particularly to transmission lines used on a probe card for electrically conductive connection to contact pads on a relatively small integrated circuit chip.
2. Description of the Prior Art
IC (integrated circuit) chips provide the function of a relatively large number of electrical components in a relatively small size. IC chips are fabricated on thin semiconductor wafers inches in diameter. Hundreds of identical IC chips are made on each wafer. Before sawing the wafer into individual IC chips for packaging, it is desirable to identify any defective IC chips by probing each chip and functionally testing it. This step eliminates the need to package any defective chip and significantly reduces the cost because the major portion of the finished part cost is in the packaging.
Conventional probing for conventional low-frequency chips is done with a probe card which is a PC (printed circuit) board with needle tipped probe blades dispersed radially in a circular hole on the PC board. Metal lines on the board connect the blades electrically to edge connectors which serve as the interface to test equipment.
Because of the uncontrolled characteristic impedance of the needle tipped blade and the metal line structure, such probe cards cease to be useful at signal frequencies above 100 MHz. For high speed circuits, such as GaAs (gallium arsenide) ICs, operating at 1000 MHz and beyond, different kinds of probes must be used.
High frequency probe circuits are available to increase the operating frequencies. In one such circuit a coaxial probe card has a number of miniature coaxial signal lines of a desired characteristic impedance, normally fifty ohms, arranged in a radial configuration. At the end of the line a needle is attached to each of the cable center conductors and to the cable outer shield for contacting the signal and ground pads on the IC chip. This approach entails high assembly cost and is not compatible with the fabrication technology of the conventional probe card.
A second approach is a coplanar IC probe which uses a ceramic substrate like alumina on which tapered coplanar transmission lines of a desired impedance are deposited. Tiny metal balls are placed at the ends of these lines to serve as contacts to the chip. The transmission line is provided almost all the way to the chip, lessening electrical discontinuity and improving high frequency performance to as high as 18 GHz. However, there are several problems associated with this approach. First, the substrate is not as flexible as conventional needles so that lack of planarity either on the IC chip or the probe would create contact problems. Furthermore, the fine line geometry requires the use of a brittle ceramic substrate. As a result, precision fixtures must be used. Secondly, the substrate blocks the view of the IC chip making it difficult for the operator to position the probe accurately on the chip pads. Thirdly the metal ball contacts do not have good wearability when compared to conventional needles. Lastly its manufacture is not compatible with existing card technology, thus implying higher cost.